The refining of hydrocarbon cuts has as its purpose the conversion of heavy molecules to lighter molecules and to remove a maximum amount of sulfur, nitrogen and metal heteroatomic impurities.
The sulfur and nitrogen heteroatoms are generally removed respectively as hydrogen sulfide and ammonia. These compounds do not deactivate the catalyst and are present in the effluents.
On the contrary, the metals of the charge, in particular nickel and vanadium, deposit at the catalyst surface generally as sulfides, thus resulting in a substantial and difficult to reverse deactivation of the catalyst, which catalyst progressively becomes inefficient in hydrodesulfurization and hydrodenitrogenation reactions.
Processes for hydrotreatment of oil fractions, particularly of fractions distilling below 550.degree. C., are well known in the art. The operations are generally conducted under hydrogen pressure, in the presence of such catalysts as molybdenum, tungsten, nickel and cobalt oxides and sulfides, for example on alumina, at temperatures generally ranging from 250.degree. C. to 450.degree. C.
Numerous searches have readily shown in particular that, by adjustment of the pore texture of conventional hydrorefining catalysts, it was possible to clearly improve the level of activity in hydrodemetallation and in conversions of heavy molecules. This modification of catalyst further provides for a substantial increase of the catalyst life (period during which the catalyst under operation is sufficiently active and thus does not have to be replaced by a new charge of fresh catalyst). Many patents or patent applications such as U.S. Pat. No. 4,395,329, U.S. Pat. No. 4,225,421, U.S. Pat. No. 4,166,026, U.S. Pat. No. 4,134,856 and No. EP-A-98 764 claim this improvement. The catalysts disclosed in the patent application No. EP-A-98 764 have a particular pore texture and their structure looks like a heap of spiky chestnut shucks or a heap of seaurchins. They are particularly efficient in demetallation of heavy oil fractions and may be used on an industrial scale.
However, such catalysts, used alone, do not provide, on the one hand, for sufficient hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) activities and, on the other hand, their activities in HDS, HDN hydrodemetallation (HDM) and heavy molecules conversion, decrease with time as a result of the poisoning of the active phase by an accumulating vanadium and nickel deposit. This insufficient stability in hydrorefining activity of these catalysts generally requires the use of a second hydrotreatment step in order to obtain acceptable final products or products useful as a charge in catalytic cracking, hydrocracking or steam-cracking units. The use of such a second step is very costly since it requires very heavy investments, particularly in materials.